Electrophilic Activation of Osmium-Nitrido Corroles: The OsN Triple Bond as a π-Acceptor Metallaligand in a Heterobimetallic OsVIN-PtII Complex.

Presented herein is a first investigation of the chemical reactivity of osmium-nitrido corroles, which are known for their unusual thermal, chemical, and photochemical stability. Elemental chlorine perchlorinates the β-positions of the triarylcorrole but leaves the OsN unit untouched. The OsN unit is also unaffected by a variety of other electrophilic and nucleophilic reagents. Upon photolysis, however, the anion of Zeise's salt associates with the nitrido ligand to generate an OsVI≡N-PtII complex. The very short OsN-Pt linkage [1.895(9)-1.917(8) Å] and the downfield 195Pt NMR resonance (-2702 ppm) suggest that the OsN corrole acts as a π-accepting ligand toward the Pt(II) center. This finding represents a rare example of the successful photochemical activation of a metal-ligand multiple bond that is too kinetically inert to exhibit any appreciable reactivity under thermal conditions.

The 5d metallocorroles are size-mismatched assemblies of a large 5d transition metal ion and sterically constrained, macrocyclic corrole ligands.[1,2] Remarkably, in spite of the steric mismatch inherent in their structures, the majority of such complexes are unusually stable, both thermally and photochemically.[3−12] In addition, many of them exhibit room-temperature NIR phosphorescence,[13−18] which has led to applications in oxygen sensors,[14−16] photodynamic therapy,[17,19] and dye-sensitized solar cells.[17] Their chemical reactivity, in contrast, remains poorly explored. Thus, gold triarylcorroles have been β-perbrominated[20,21] and certain MVO corroles have been derivatized to the corresponding Viking-helmet dichlorides, that is, MVCl2 corroles.[22] In this communication, we document two complementary avenues for the further elaboration of osmium(VI)-nitrido corroles (Scheme ). One involves direct reaction with elemental chlorine, affording a facile route to OsVIN octachlorocorroles. The second is more innovative and is based on the use of the OsVIN moiety as a π-acceptor ligand in the formation of an OsVIN–PtII heterobimetallic complex, as described herein.

Scheme 1

Electrophilic Activation of OsN Corroles

Unfortunately, β-perbromination of OsVIN triarylcorroles proved difficult: many hours of exposure to elemental bromine led only to a mixture of the hexa-, hepta-, and octa-brominated products. Although steric congestion in the products is a tempting explanation, we believe that the high-oxidation-state Os exerts the key deactivating influence on the corrole macrocycle. In contrast, β-perchlorination of OsVIN corroles occurred readily upon exposure to elemental chlorine at room temperature for several minutes. Since we were primarily interested in metal-centered reactivity, we characterized and structurally analyzed only one β-perchlorinated product, Os[Cl8TPC](N) (1, Table S1 and Figure ). The Os–Ncorrole (1.98–2.00 Å) and Os–Nnitrido (1.63–1.64 Å) distances proved similar to those found in β-unsubstituted OsVIN corroles.[6] Both the Soret and Q bands of Os[Cl8TPC](N) were found to be significantly red-shifted relative to the starting material, reflecting the electronic effect of β-octachlorination (Figure ).

Figure 1

Thermal ellipsoid (50%) plot for Os[Cl8TPP](N) (1). Selected distances (Å): Os1–N1A 1.986(5); Os1–N2A 1.999(4); Os1–N3A 2.004(5); Os1–N4A 1.979(5); Os1–N5A 1.630(6); Os2–N1B 1.991(5); Os2–N2B 1.990(5); Os2–N3B 1.994(5); Os2–N4B 1.990(5); Os2–N5B 1.642(5).

Figure 2

Comparative UV–vis spectra in dichloromethane: (a) Os[Cl8TPC](N) (1) and Os[TPC](N); (b) Os[TpCF3TPC](N) and (AsPh4){Os[TpCF3TPC](N)-PtCl3} (2).

Osmium-nitrido complexes represent the border between nucleophilic and electrophilic reactivity,[23−36] which are characteristic of early and late transition metal-nitrido complexes, respectively. Yet, a preliminary examination of the reactivity of the OsVIN corroles with both nucleophilic (PPh3) and electrophilic reagents (MeI, I2) failed to yield the expected nitride-derived products. Taking a cue from recent uses of nitride[37−39] and carbide[40,41] complexes as novel π-acceptor ligands, we attempted to deploy OsVIN triarylcorroles in a similar manner. No transformation, however, was observed upon exposure to electron-rich transition metal complexes with labile ligands, including AuCl(tht), [RhCl(cod)]2, [IrCl(cod)]2, and the Zeise’s salt analogue (AsPh4)[Pt(C2H4)Cl3] (tht = tetrahydrothiophene, cod = 1,4-cyclooctadiene).[37−41] Indeed, even upon prolonged heating in bromoform (100 °C, 20 h), the OsVIN corroles and (AsPh4)[Pt(C2H4)Cl3] failed to react. As a last resort, we attempted to activate OsVIN corroles photochemically (λ = 365 nm), inspired in part by the reports of photolytic activation of OsVIN complexes leading to N–N coupling[42] and C–H functionalization.[43] The results proved promising, especially with the electronegatively substituted tris(p-trifluoromethylphenyl)corrole complex Os[TpCF3TPC](N). According to NMR analysis, UV irradiation with [RhCl(cod)]2 over 18 h led to low levels (7%) of conversion to Os[TpCF3TPC](N)–Rh(cod)Cl, as evidenced by an upfield shift of the β-hydrogen resonances of the corrole. Much better yields of (AsPh4){Os[TpCF3TPC](N)–PtCl3} (2, 30% conversion after 20 h) were obtained with (AsPh4)[Pt(C2H4)Cl3] in dichloromethane. The retention of the 1H resonance from unconverted Pt(C2H4)Cl3– (2JPt–H ≈ 60 Hz) suggests that the photolytic reaction proceeds via activation of the OsVIN corrole, as opposed to a scenario in which the ethylene ligand dissociates to generate an activated platinum(II) center.

The X-ray structure of 2 (Table S1 and Figure ) revealed a slight elongation (∼0.02 Å) of the Os≡N triple bond relative to the starting complex.[6] Comparison of the different Pt–Cl distances showed that the nitride ligand in 2 exerts a slightly weaker trans influence than chloride (2.295 versus 2.318 Å), indicating that the OsVIN corrole is a weak σ-donor. Remarkably, the OsN–Pt bond proved to be much shorter (∼1.90 Å) than those in most other [Pt(L)Cl3]− anions with N-donor ligands (Table S2), indeed among the shortest 1% in the Cambridge Structural Database (Figure S9). Such a short Pt–N bond suggests partial multiple-bond character, consistent with the Os≡N triple bond acting as a π-acceptor ligand toward Pt(II), which in turn is consistent with dramatic changes in the UV–vis spectrum upon Pt(II) coordination (Figure ) as well as with 195Pt NMR spectroscopy.[44,45] In the latter method, a π-backbonding ligand results in a decrease of the electron density at the platinum nucleus and thereby a downfield shift of the 195Pt resonance. Complex 2 exhibits a 195Pt NMR chemical shift at −2702 ppm, downfield of the starting material (AsPh4)[Pt(C2H4)Cl3] at −2751 ppm. To gain a wider frame of reference, we also recorded the 195Pt NMR spectrum for a carbide-bridged analog of 2, (AsPh4)[(Cy3P)2Cl2Ru≡C–PtCl3], which exhibits a strongly downfield-shifted 195Pt NMR resonance at −2499 ppm.[40] Thus, the π-backbonding strength of L in the (AsPh4)[Pt(L)Cl3] family increases in the order L = C2H4 < Os[TpCF3TPC](N) < (Cy3P)2Cl2Ru≡C, thereby establishing Os[TpCF3TPC](N) as a moderately strong π-backbonding ligand. Differences in π-backbonding between M≡C and M≡N metallaligands presumably impact their relative lability; thus, Ru≡C ligands are less prone to dissociate from Pt(II) centers than Cr≡N ligands.[46]

Figure 3

Thermal ellipsoid (30%) plot for (AsPh4){Os[TpCF3TPC](N)-PtCl3} (2). Selected distances (Å): Os1–N1 1.664(9), Os1–N2 1.979(9), Os1–N3 1.988(8), Os1–N4 1.978(9), Os1–N5 1.969(8), Pt1–N1 1.895(9), Os2–N6 1.656(8), Os2–N7 1.969(8), Os2–N8 1.982((8), Os2–N9 1.978(7), Os2–N10 1.973(7), Pt2–N6 1.917(8).

In conclusion, OsVIN corroles are surprisingly inert toward both nucleophilic and electrophilic reagents. Elemental chlorine perchlorinates the β-positions of the corrole but leaves the central OsN unit untouched. Only upon photolysis does the OsN unit react with the Zeise anion, affording a heterobinuclear Os≡N–Pt complex. Although mechanistic details remain to be elucidated, this study may well provide a rare example of photochemical activation of a metal–ligand multiple bond that is too inert to react thermally with common nucleophiles and electrophiles. That in turn would imply that the common classification of metal–ligand multiple bonds as nucleophilic, electrophilic, or ambiphilic may provide an incomplete picture of their reactivity; that is, excited state reactivity may differ dramatically from ground state reactivity.